The American Ornithologists' Union (AOU) is an ornithological organization. It is found in the USA. Its members are mainly professional ornithologists. It is similar to the National Audubon Society, except its members are mainly amateur birders. It is a member of the Ornithological Council.
It has a journal, called The Auk. It is published four times a year. It has been published since January, 1884. Some of its other big publications are the AOU Checklist of North American Birds and a series, Ornithological Monographs. The AOU Checklist of North American Birds is the standard reference for the field.